1. Field of the Invention
This invention relates to the recovery of metal from dross.
2. Description of the Prior Art
It is well known that when many molten metals are in contact with the atmosphere, compounds of those metals, primarily oxides, are formed. Especially when there is some movement of the molten metal, the metal and the metal oxide combine to form a material known as dross. Dross consists of droplets of the molten metal which are encrusted with the oxide which forms a sponge-like network. The dross of lead-tin solder, for example, appears like rather fibrous demerara sugar. The dross floats, due to surface tension and/or buoyancy, on the molten metal or sticks to the container holding the metal.
In many industrial processes where dross is formed, it is necessary to remove the dross from time to time. For example, in a wave soldering machine, solder in a bath is pumped to create a static wave in the surface of the solder, and printed circuit boards are fed across the crest of the wave so that the solder kisses the leads of the components and the tracks of the circuit board. If the dross is allowed to build up, it can become entrained in the wave and adversely affect the quality of the soldering, causing low product yields.
The dross could simply be ladled out from the molten metal and disposed of. However, the metal content of the dross can be high and typically, according to one estimate, is in the range of 30 to 90%. The cost of disposing of the raw dross and replacing it with "clean" metal can be a significant.
Attempts have been made in the past to recover the metal content of the dross. For example, patent documents U.S. Pat. No. 4,119,136 and U.S. Pat. No. 4,334,664 describe the recovery of tin from tin dross by draining the tin out at high temperature. The recovery of aluminium from aluminium dross is described in, for example: U.S. Pat. No. 4,772,320 (squeezing hot dross between rollers under pressure); U.S. Pat. No. Re. 31,028 (rolling and milling of cold dross); U.S. Pat. No. 4,394,978 (grinding and screening of cold dross); and U.S. Pat. No. 4,386,956, U.S. Pat. No. 4,540,163, U.S. Pat. No. 4,565,572, WO-A-82/01,895 and WO-A-84/03,719 (squeezing of hot dross between a ram or piston and a trough or cylinder). The recovery of lead-tin alloy from solder dross is described in WO-A-95/25,823, and this also involves squeezing the hot dross with a piston in a cylinder. These latter "hot-squeeze" techniques require the use of a large and substantial machine in order to apply the required pressure, and this in itself is expensive. In existing soldering production lines, there is often not much spare space near the soldering bath, and therefore it may be necessary to site such a large machine away from the soldering bath, as a result of which the dross needs to be reheated before it can be squeezed. Also, the hot-squeeze machines, as exemplified by WO-A-95/25,823, operate on an essentially five-phase cycle: (1) adding the dross to the machine; (2) raising the dross to the required temperature; (3) squeezing the dross; (4) allowing the metal to drain from the dross; and (5) removing the oxide. (Also, it may periodically be necessary to clean a grille or sieve through which the recovered metal or oxide passes.) Accordingly, dross cannot be added to the machine as and when required, only during phase "1" of the cycle.